<!DOCTYPE html>
<html lang="zh-cn">
    <head>
        <meta charset="UTF-8"/>
        <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
        <title>Document</title>
        <style>
            *{
                margin: 0;
                padding: 0;
            }
            html,body{
                height: 100%;
            }
            .box-video{
                height: 40%;
                width: 100%;
                background-color: #fc0;
            }
            .box-bottom{
                position: relative;
                height: 60%;
            }
            .box-comment{
                position: absolute;
                top: 0;
                left: 0;
                bottom: 40px;
                width: 100%;
                overflow:auto;
                background-color: #f0c;
            }
            .box-input{
                position: absolute;
                left: 0;
                bottom: 0;
                height: 40px;
                width: 100%;
                background-color: #cf0;
            }
        </style>
    </head>
    <body>
        <div class="box-video">video</div>
        <div class="box-bottom">
            <div class="box-comment">评论框</div>
            <div class="box-input">
                <input type="text" placeholder="请输入评论"/>
            </div>
        </div>
    </body>
</html>